Field of the Invention
The present invention relates to the field of security marking. More particularly, it relates to an optical security component with a reflective effect which is used to verify the authenticity of a document, and more particularly the authenticity of fiduciary documents such as banknotes.
Related Art
Security threads are a reliable and effective security element for the protection of banknotes against counterfeiting. Incorporated into the paper when the banknotes are manufactured, they form a windowed thread visible on the surface of the paper at regular intervals. Due to their complex manufacturing technique, they offer a high level of protection against counterfeiting. Security threads can reach a width of 5 mm and have multi-functional properties. In particular, they may contain information identifiable by everyone, such as a pattern, drawings, alphanumeric characters or repeating signs. They may also comprise optical security elements for authentication of a banknote, for example producing holographic effects. These optical security elements must be such that they can withstand the manufacturing constraints of the banknotes, in particular the pressure or temperature constraints relating to the drying or printing phases of the banknote.
Patent FR 2,838,202, granted in this applicant's name, describes an optical security component compatible with the production of security threads and observable by direct reflection. One such component is described in FIG. 10A. It comprises a layer made from a transparent dielectric material with a high refractive index (104) encapsulated between two low-index layers (103, 105), and structured to form a subwavelength grating. One such component, called DID (Diffractive Identification Device), behaves like a structured waveguide making it possible to excite the resonance modes which are guided at different wavelengths as a function of the polarization, the incidence angle and the azimuth. The principle of such an effect is also described within the published patent application FR 2,509,873. By direct reflection (diffraction of order 0), such a component thus acts as a wavelength subtractive filter, or pass band filter, forming a colored mirror wherein the color varies with the viewing direction. In order to increase the visibility of the effect of the orientation changes of the component, described in patent FR 2,838,202 there is the arrangement of a colored contrast layer 102 having transparent zones and colored zones, the colored zones being capable of assuming identifiable shapes.
Patent application WO 2006/038120 also describes an optical security component, one example of which is illustrated in FIG. 10B. The described component comprises two layers 121, 122 with a high refractive index, each encapsulated between layers with a low refractive index (123, 124 and 124, 125, respectively) and structured to form two observable DID type, zero order diffraction gratings. The arrangement of two superimposed gratings makes it possible to reinforce the visible variable color effects by rotating the component. The application for a security thread comprising identifiable information is not, however, disclosed.
Patent application WO 2011/104551 also describes an optical security component, one example of which is illustrated in FIG. 10C. The component described here comprises transparent colored elements (150, 154) for example made from a UV cross-linkable resin upon the surface of which microstructures 156 are formed that are capable of generating variable optical effects. A reflective layer 130 extends over all of the elements supporting the microstructures and within the laterally offset regions. Using discontinuous colored elements makes it possible to cut between the different elements in order to form multiple components more easily during the manufacturing method. Furthermore, using resins with different colors for a same component makes it possible to produce effects of different colors. The component described is applicable to security threads, but the search for a greater contrast between the different regions is not considered.
The present invention describes an optical security component that can be inspected by zero order reflection and with the naked eye by an observer, said component allowing, with respect to the components disclosed in the prior art, increased contrast regarding the information identifiable by a viewer, in particular the information borne by the security threads of the banknotes. The invention also relates to a method for producing one such component.